Polyamides typified by Nylon 6, Nylon 66 have been widely used as various component materials for clothes, industrial materials, automobiles, electric and electronics, engineering and other purposes for their excellent moldability, mechanical physical properties, and chemical resistance.
Recent demands for, for example, metal substitution for lighter components however, have led to increasing demands for further improvement in mechanical properties and functions such as fuel barrier resistance and coolant resistance in polyamide molded products. More specifically, conventional Nylon 6 and Nylon 66 are not sufficient in heat resistance, dimensional stability, mechanical properties, chemical resistance and other properties when used for fuel tubes, coolant hoses and air intake ducts for automobiles; therefore, the current situation is that they cannot be used in such applications.
In response to such demands, semi-aromatic polyamides have been proposed in order to solve such problems pertinent in conventional polyamides. As the semi-aromatic polyamides, for example, high melting point semi-aromatic polyamides including terephthalic acid and 1,6-hexanediamine as the main components (hereinafter simply referred to as “6T-based copolyamides” in some cases) have been proposed, with some of which being in practical use.
A polyamide consisting of terephthalic acid and 1,6-hexanediamine (hereinafter, simply referred to as “PA6T” in some cases) is a polymer having a melting point of as high as about 370° C. Therefore, when a molded article is to be obtained by melt-molding of PA6T, the polymer undergoes significant thermal degradation, and therefore, a molded article having sufficient properties cannot be easily obtained.
Thus, 6T-based copolyamides have been used which are obtained by copolymerization of PA6T with aliphatic polyamides such as Nylon 6 and Nylon 66 or with amorphous aromatic polyamides such as Nylon 61 to reduce the melting point to a level as low as about 220 to about 340° C. These 6T-based copolyamides do exhibit such properties as low water absorption property, high heat resistance and high chemical resistance, but exhibit low melt tension; therefore, the shape of their parison cannot be easily retained during extrusion molding or blow molding and drawdown easily occurs.
Thus, 6T-based copolyamides have been used which are formulated with large amounts of resin other than polyamide resins, such as polyphenylene ether resins, polyolefin resins or modified polyolefin resins for improved blow moldability (see, e.g., PTLs 1 and 2). However, due to the influence of the presence of large amounts of resins other than polyamides, these 6T-based copolymers have the drawback of reduced 6T-based copolymer's properties inherent in polyamide resins, such as mechanical strength, heat resistance, and chemical resistance.
On the other hand, 6T-based copolyamides not blended with other resins are hardly subjected to extrusion molding or blow molding.
Nylon 12, which has superior flexibility, is frequently used in fuel piping for automobiles. The transpiration regulation of fuel gas with respect to automobiles, however, has been increasingly severer in recent years, and various low-permeable fuel hoses have been studied that would meet the transpiration regulation.
While the fuel tubes have been conventionally made of fluororesin, such fuel tubes have attracted attention that are made of resin that is more inexpensive and more excellent in fuel low permeability than fluororesins, e.g., polyphenylenesulfide resin (PPS), aromatic polyamide resin such as polyamide 6T (PA6T) or polyamide 9T (PA9T), or polyester resin such as polybutylene terephthalate (PBT).
Various tubes provided with a fuel-low permeable layer made of such a resin have been proposed (see, e.g., PTLs 3 to 6). Nevertheless, the resins that are replacing fluororesins and attracting attention have high rigidity, poor flexibility, and low impact resistance particularly at low temperatures, and thus tubes made of these resins are easily cracked. On the other hand, piping for automobiles, such as fuel system piping and cooling system piping, is commonly produced by blow molding or extrusion molding. General polyamide resins have low melt tension and therefore the shape of their parison is cannot be easily retained during extrusion molding or blow molding and thus drawdown easily occurs. Therefore, almost no examples have been reported in which a polyamide resin not blended with other resins is subjected to extrusion molding or blow molding.
Polyamides typified by Nylon 6 and Nylon 66 are also used for basic engine components such as engine covers, connectors connected thereto and air intake manifold, and automobile components such as relay boxes, gears and clips. Meanwhile, recent reductions in the size of the engine room and increases in the engine performance and engine output have caused a rise in the temperature of the engine room and engine cooling water. Thus, resin products used in these applications have been increasingly required to have higher heat resistance, and there is a growing demand for polyamides that exhibit excellent heat resistance compared to Nylon 66.
As described above, crystalline polyamide resins typified by Nylon 6 and Nylon 66 generally have low melt viscosity, and therefore the shape of their parison cannot be easily retained during blow molding and drawdown easily occurs. Therefore, there are cases wherein measurement of the mass of parison and/or control of the thickness of a product is difficult. To avoid this problem, as disclosed in PTL2, a method is proposed wherein a crystalline polyamide resin is formulated with a modified olefin resin such as an ionomer resin. This method results in a slight increase in the melt viscosity and a decrease in thickness variation of a small blow-molded article. However, since a relatively long parison is used in advanced blow moldings in which products having a complicated shape or many inserting components are integrally molded, such as three dimensional blow molding, multidimensional extrusion blow molding and multilayer molding, drawdown easily occurs and a uniform product cannot be molded with this method. This method also has the drawbacks of reducing mechanical strength and chemical resistance of a molded article due to the addition of large amounts of olefins or ionomers.
To avoid this problem, a method has been proposed wherein a crystalline polyamide resin is formulated with glass fiber or the like to increase apparent melt viscosity (PTL 7). This method, however, has the drawback of instable melt viscosity, and drawdown easily occurs particularly in a large blow-molded article, and thickness variation is large in a blown product.
Another method has been proposed wherein a polyamide resin consisting of a decane terephthalamide unit and an undecaneamide unit is formulated with a reactivity modifier (glycidyl group-containing styrene-based polymer or carboxylic acid group-containing olefin-based polymer) and a fibrous reinforcing material with (PTL 8). With this method, however, it is difficult to control the reaction between the polyamide resin and the reactivity modifier, and thus local thickening may occur during the retention of resin resulting in thickness variation in a molded article. Moreover, a crosslinked gel may be formed that deteriorates the appearance of a molded article. Furthermore, this method has the serious drawback of reducing mechanical strength of a molded article due to incorporation of long-chain methylene moiety in the main chain for increased melt viscosity of the polyamide itself.